1. Field of the Invention
The present invention relates to a vulcanizing mold for producing pneumatic tires.
2. Description of Related Art
In vulcanizing tires having deep grooves, there have been widely used so-called segmented molds. As generally know in the art, the segmented mold includes a plurality of mold segments that can be combined circumferentially with each other to define an inner surface for forming the entirety of the tread portion of the tire, as well as upper and lower side mold members for forming both side portions of the tire. The upper side mold member is secured to a top plate, and is movable axially of the mold, toward and away from the lower side mold member. The mold segments are arranged in, and slidingly engaged with a container ring so that they are moved radially inwards or outwards of the mold as the container ring is moved vertically downwards or upwards.
In such a segmented mold, the mold segments are each provided with an inner peripheral surface that extends over the entire width of the tread portion of the tire, and are moved radially outwards when the mold is opened to remove the vulcanized tire. Thus, the mold can be opened without interference between the ridges on the inner surface of the mold segment and corresponding grooves in the tread portion of the tire, thereby avoiding formation of defective tread portion. However, since the mold segment corresponds to the entire width of the tread portion, it is inevitable that the segment is relatively large and heavy. Moreover, in order to ensure that the mold segments are moved radially to precisely form the mold cavity without any inconvenience, the sliding-engagement surface of the container ring for guiding the mold segments must have a sufficient length. Thus, it is also inevitable that the container ring supporting the segments is large, resulting in increased size and cost of the mold as a whole.
When it is contemplated to use a segmented mold for producing particularly large tires, such as those for construction vehicles, it is often difficult to accommodate the mold in existing vulcanizing machine, and it is thus necessary to prepare an entirely new vulcanizing machine.
It is possible to vulcanize pneumatic tires having deep grooves in the tread portion, by using an ordinary vulcanizing mold, i.e., so-called two-split mold without segments and container ring, which can be readily accommodated in existing vulcanizing machine However, there is a serious problem that, when the vulcanized tire is removed from the vulcanizing mold, defective tread portion is caused due to interference between ridges on the inner surface of the mold and corresponding grooves in the tread portion. Thus, two-split mold cannot be used to produce large pneumatic tires with deep grooves in the tread portion.
It is an object of the present invention to eliminate the above mentioned problems of the prior art, and provide a vulcanizing mold which is compact and inexpensive, and which is capable of producing large tires having deep grooves in the tread portion while positively avoiding formation of defective tread portion.
According to the present invention, there is provided a vulcanizing mold for producing a pneumatic tire comprising a tread portion, wherein the tread portion comprises a center region which includes an equatorial plane of the tire, and outer regions on both sides of the center region, and wherein the center region and outer regions of the tread portion are bounded by two planes perpendicular to a center axis of the tire. The mold according to the present invention comprises a pair of side mold members arranged opposite to each other and movable at least axially of the mold toward each other to define part of a mold cavity in which a green tire is vulcanized, and away from each other to allow setting of a green tire into the mold cavity and removal of a vulcanized tire out of the mold cavity, wherein each side mold member has an inner peripheral surface provided with ridges corresponding to grooves in the outer region of the tread portion of a tire. The mold according to the present invention further comprises a plurality of distance piece arranged axially between the side mold members and movable radially of the mold, wherein the distance pieces are circumferentially combined with each other when they are moved radially inwards, to form an annular inner peripheral surface provided with at least one ridge corresponding to at least one groove in the center region of the tread portion of the tire. The ridges of the side mold members extend in parallel with each other, as seen in a developed view of the inner peripheral surface of the side mold member, and terminating at positions corresponding to a tread end of the tire so that the grooves formed by these ridges are opened in the tread end.
With the vulcanizing mold according to the present invention, the plurality of distance pieces are moved radially inwards and thereby combined with each other to provide an annular inner surface corresponding to the center region in the tread portion of the tire. Thus, by moving these distance pieces radially outwards upon opening the mold, the at least one ridge on the inner surface of the distance pieces can be separated radially away from the tire that has been vulcanized, without interfering with the tread portion of the tire.
Furthermore, the side mold member for the outer region of the tread portion has ridges corresponding to lug grooves of the tire, which is opened at the tread end of the. Thus, by moving the side mold members away from the tire in a direction along the lug grooves, it is possible to open the mold without interference of the ridges of the mold with the tire. In this way, the mold according to the present invention makes it possible to remove the vulcanized tire from the mold without interference between the ridges of the mold and the tire.
The distance pieces in the mold according to the present invention have a narrow and compact structure since they correspond exclusively to the center region of the tire. This makes it possible to realize a compact mechanism for moving the distance pieces, thereby providing a mold at a minimized size and cost comparable to those of conventional two-split mold.
The at least one ridge of the distance pieces may extend in circumferential direction of the mold to form at least one circumferential groove in the center region of the tread portion of the tire.
The ridges of the side mold members may each extend along a meridian of the tire. Alternatively, the ridges of the side mold members mat each extend helically about a center axis of the tire, with a predetermined pitch.
It is preferred that the mold further comprises guide means for guiding the movement of the side mold members away from each other when opening the mold, and positioning the side mold members relative to each other when closing the mold In this way, the relative position of both side mold members is accurately defined by the guide means upon closing the mold, thereby precisely forming the mold cavity of a predetermined shape. Further, the side mold members are properly guided and separated from each other along the direction of the ridges on the inner perpheral surface of one of the side mold members, upon opening the mold. Thus, the side mold members can be separated from the tire without applying undesirable stresses to the tire, thereby positively preventing damages to the tire or formation of defective tread portion. The ridges of the side mold members are provided such that they are drawn out along the corresponding lug grooves of the tire when the side mold member is separated from the tire. However, there is applied a larger separating force to the tire, at least in the initial stage of the mold opening. In the absence of the guide means, the lug grooves of the tire would have to guide the side mold members against such larger force, thereby applying substantial force to the tread portion of the tire and forming defective tread portion, as the case may be. Provision of the guide means makes it possible to eliminate or mitigate stresses applied to the tire, so that damage of blocks are rarely caused.
It is preferred that the guide means comprises at least one wedge provided for one of the side mold members, and at least one notch provided for the other of the side mold members, said wedge being engageable with said notch. In this instance, it is readily possible to realize a simple and reliable structure of the guide means.
It is preferred that the mold according to the present invention further comprises biasing means for normally biasing said distance pieces radially outwards. In this way, it is unnecessary to separately provide actuator means, such as air cylinders, for moving the distance pieces radially outwards when the mold is opened and the vulcanized tire is removed, so that the mold can be made more compact.
It is preferred that the distance pieces are engageable with one of the side mold members so that the distance pieces are moved radially of the mold upon axial movement of that side mold member. In this instance, the side mold member may have a cam surface engaged by the distance pieces so that the distance pieces are moved radially inwards as the mold member is moved axially toward the other side mold member.